In the following discussion, reference is made to FIG. 1. As is known, two conductor measuring devices are measuring devices that are equipped with only two conductors L1, L2, which enable them to be electrically connected from and to the outside. These two conductors must be used both for supplying energy and for transmitting a measuring signal generated by the measuring device. The measuring signal values produced by the two conductor measuring device are direct currents within a standardized range of 4 mA to 20 mA, such that one current value within this range corresponds precisely to one measuring signal value.
The above-mentioned supplying of energy is accomplished by means of a direct current source that, during operation, is connected to the two conductors from the outside, generates a direct current, and belongs to a primary network NP, so that a so-called current loop is created. This loop also contains a current measuring resistor Rm, where a current that is proportional to the current value at any given time, and thus to the measuring signal, can be tapped and further processed.
The resistor Rm can be positioned a great distance from the two conductor measuring device MS, in this case it is connected to the measuring device via cables of appropriate length. Specifically, the terminal NP1 of the primary network NP is connected to the conductor L1 of the two conductor measuring device MS′, and the terminal NP2 is connected to one port of the resistor Rm, while the resistor's other port is connected to the conductor L2 as an indirect second primary network port NP2′, as it were.
In addition to the above-mentioned current values, which are analog signals, digital signals can also be transmitted at the two conductors L1, L2, in accordance with one of the customary standards, such as the so-called HART protocol. The HART protocol (HART is a registered trademark of the HART User Group and is an acronym for “Highway Addressable Remote Transducer”, in other words for bus-addressed measuring devices) has long been known and used in industrial measuring technology.
The HART protocol enables communication between a field level and a process control level, providing the advantage of simultaneous transmissibility of an analog measuring signal in accordance with the 4-mA to 20 mA standard, and of the digital HART signal for start-up, maintenance, polling, or control of the measuring devices in the field level.
While the analog measuring signal is continuously available, cyclical polling and, if necessary, a subsequent instruction via the digital HART signals takes place. In this process, a digital zero is realized via two sine-wave oscillations at a frequency of 2.2 kHz, and a digital one is realized via a single sine-wave oscillation at a frequency of 1.2 kHz, in keeping with the standard Bell 202 Frequency Shift Keying. These sine-wave oscillations are transmitted via the two conductors, in that they are modulated upon the current flowing therein.
Current state-of-the-art two conductor measuring devices are competitive, in other words marketable by the manufacturer, only if they have been tested for electromagnetic compatibility (abbreviated: EMV). The testing must be conducted in compliance with the currently valid international standard IEC-1000-4-5:1995, which has been adopted in individual countries to correspond with national standards, and is a so-called type test.
In a type test, each individually produced device is not tested. Instead, the testing of one or a few devices from a group of identical devices is sufficient.
The standard IEC-1000-4-5:1995 has been valid since 1995. According to the previous version, which was valid until 1995, the two conductors L1, L2 were considered only as signal lines used to transmit measuring signals, and thus were not subject to the more rigorous specifications for energy supply lines, for which the conductors were tested.
In 1995, however, this approach and classification was tightened, with the direct connection of the two conductor measuring devices to the direct current source G and/or its primary electrical system NP; the two conductors L1, L2 now are no longer defined merely as signal lines, but also as supply lines, hence they are subject to the above-named rigid test specifications and are tested on that basis.
These test specifications require that the primary electrical system that serves to supply energy and originates from the direct current source Q also generate high-energy interference pulses in addition to the direct current, which a two conductor measuring device MS′ that is connected to the network must withstand.
These test specifications simulate real conditions such as occur in practice when two conductor measuring devices e.g. are to be used for means of transport, especially in ships. In a primary vehicle electrical system NP installed on a ship, the above-mentioned interference pulses are frequently present, (FIG. 1). Hence, the two conductors L1, L2, in state-of-the-art two conductor measuring devices must be tested using the test signals based upon the standard IEC-1000-4-5:1995 for energy supply lines, hence they are to be dimensioned to be resistant relative to these test signals.
This does not make sense in practical terms, however, since it would mean that in the two conductor measuring device MS′, which is equipped with a standardized case Gh, electronic components, i.e. a conductance coil L′ and a condenser C′, would have to be incorporated at the input side in a suitable combination, serving to suppress or at least filter the interference pulses that occur in the primary vehicle electrical system NP.
These incorporated elements, however, increase production costs. Furthermore, the dynamic space SF required for these filter components, which in most cases must be installed in multiples, is not available, so that the size of the standardized casing Gh must be increased by a casing volume Gz, which increases costs substantially. Finally, the applicability of the above-mentioned HART protocol is severely curtailed as a result of the filter components, and in some cases is rendered impossible.